The coffin was discovered inside a much larger limestone sarcophagus during a second excavation of the site, in August 2013 - one year after the remains of the former King of England were unearthed.

Richard III will be reinterred at Leicester Cathedral this month after his mortal remains are taken from the University of Leicester.

Inside the lead coffin, archaeologists found the skeleton of an elderly woman, who academics believe could have been an early benefactor of the friary - as radiocarbon dating shows she might have been buried not long after the church was completed in 1250 (although analysis shows her death could have taken place as late as 1400).

The high status female was in one of 10 graves discovered in the grounds of the medieval complex, including that of Richard III, six of which were left undisturbed. Those that were examined were all found to have female remains.

"Although it might seem unusual that Richard III is the only male skeleton found inside the Grey Friars church, the other four skeletons all being female, it must be remembered that we have only excavated five of ten identified graves in the church's chancel with the potential for hundreds more burials elsewhere inside the church, the other friary buildings and outside in the cemetery," Grey Friars site director Mathew Morris, who led the dig said.

"Excavations of other monastic cemeteries have found ratios ranging from 1:3 to 1:20 woman to men buried, with urban monastic cemeteries typically having greater numbers of women buried in them than rural sites," said Morris.

"Richard III would certainly not have been the only male buried here during the friary's 300 year history and historic records list at least three other men buried in the church," said Morris.

"What stands out more is the contrast between the care and attention taken with these burials-- large, neatly dug graves with coffins-- and the crudeness of Richard III's grave. The more we examine it, the clearer it becomes how atypical Richard III's burial really was," Morris said.

The lead coffin, with an inlaid crucifix, the location of her burial in presbytery of the friary's church (possibly close to the high altar) meant that she had a special significance to the holy Catholic order, researchers said.

The discovery is the first example of an intact medieval stone coffin to be unearthed in Leicester during modern excavations, they said.